1. Field of the Invention
This invention relates to an active inductor, and more particularly to an active inductor in which a field effect transistor is employed.
2. Description of the Background Art
In monolithic microwave integrated circuits (hereinafter referred to as xe2x80x9cMMICsxe2x80x9d), it is required to obtain an impedance matching between circuits and to improve the bandwidth of circuits. To meet these requirements, a capacitor and an inductor are used in MMICs. As an inductor for use in MMICs, a spiral inductor has been employed in many cases which is made of a metal conductor wound in a spiral manner on a dielectric substrate.
A spiral inductor has a simple construction, whereas its configuration has to be enlarged in order to obtain a large inductance. Thus, a spiral inductor practically tends to occupy a large area. Moreover, there have been problems in that a resistance component is high, resulting in an increase in power consumption and that cross-talk occurs between a spiral inductor and a peripheral circuit thereof.
In order to solve the above noted problems encountered in a spiral inductor, there have been proposals for an active inductor using a transistor which is an active element. Advantageously, an active inductor can be miniaturized compared to a spiral inductor and, besides, a resistive component is low, which allows reduction in power consumption. Further, cross-talk rarely occurs between an active inductor and a peripheral circuit thereof. For these characteristics, an active inductor is suitable for MMICs.
Conventionally, combination of a gyrator in a four-port network circuit and a capacitor has been proposed as an active inductor, for a gyrator has a function of converting an impedance. In particular, a field effect transistor has a high input impedance, which is therefore suitable for constituting a gyrator. The technique for forming an active inductor by a capacitor and a gyrator comprising a field effect transistor is disclosed, for example, in Japanese Patent Application Laid-Open No. 4-233312.
On the other hand, another type of active inductor has also been proposed which is not so constructed as to convert an impedance of a capacitor using a gyrator. FIG. 23 is a circuit diagram showing a construction of an active inductor 200 which is introduced, for example, in xe2x80x9cBroad-Band Monolithic Active Inductor and Its Application to Miniaturized Wide-Band Amplifiersxe2x80x9d, Hara et al., IEEE Transactions on Microwave Theory and Techniques, MTT-36, No. 12, pp.1920-1924 (December 1988). Transistors M1 and M2 are both field effect transistors, each having a drain, a gate and a source represented as D, G and S, respectively. The source of the transistor M1 is connected to the gate of the transistor M2, while the drain of the transistor M1 is connected to the source of the transistor M2. A resistor R is connected between the gate of the transistor M1 and the drain of the transistor M2, and an impedance between the drain and gate of the transistor M2 acts as an inductor. A bias circuit for operating the transistors M1 and M2 is not illustrated in the figure.
Both of the field effect transistors M1 and M2 can be represented as an equivalent circuit as shown in FIG. 24. Specifically, when a parasitic capacitor Cgs is provided between the source and the gate and the gate-source voltage is Vgs, operating characteristics of the transistors M1 and M2 can be approximated by operations of the equivalent circuit having a current source for flowing current gmVgs from the drain to the source (gm greater than 0: transconductance). For simplification, symbols in the circuit also indicate values of an element, potential or current which they represent. The expression xe2x80x9capproximate a circuit by its equivalent circuitxe2x80x9d, which will be used hereinafter, represents approximation of operations of a circuit by operations of its equivalent circuit.
Employing the equivalent circuit shown in FIG. 24, numerical subscripts 1 and 2 are added to the parasitic capacitor Cgs, the transconductance gm, the gate-source voltage Vgs, respectively, so as to make clear that they are elements in the equivalent circuit of the transistors M1 and M2. This allows the active inductor 200 to be approximated by the equivalent circuit shown in FIG. 25.
The drain-gate voltage of the transistor M2 is indicated by V, and current flowing in the active inductor shown in FIG. 23 is indicated by I (let positive a direction the current flowing into a junction between the drain of the transistor M2 and the resistor R). An angular frequency of the voltage V is indicated by xcfx89, and an imaginary unit (xe2x88x921)1/2 is indicated by j. Then, admittances of the parasitic capacitors Cgs1 and Cgs2 are indicated by jxcfx89Cgs1 and jxcfx89Cgs2, respectively.
Therefore, an impedance of the active inductor shown in FIG. 23 is expressed as follows:                                                         Z              ⁢                              xe2x80x83                            =                              xe2x80x83                            ⁢                              V                                                      xe2x80x83                                    ⁢                  1                                                                                                                        xe2x80x83                            ⁢                              =                                  xe2x80x83                                ⁢                                                      1                                                                  xe2x80x83                                            ⁢                                              1                        +                                                                              (                                                          xe2x80x83                                                        ⁢                                                                                          ω                                ⁢                                                                  xe2x80x83                                                                ⁢                                                                  C                                                                                                            xe2x80x83                                                                        ⁢                                    gs2                                                                                                                                                                                                xe2x80x83                                                                ⁢                                                                  g                                                                                                            xe2x80x83                                                                        ⁢                                    m2                                                                                                                                                        )                                                    2                                                                                                      ·                                                                                                                        xe2x80x83                            ⁢                                                                    1                    +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                                              C                                                                              xe2x80x83                                                    ⁢                          gs1                                                                    ⁢                                              xe2x80x83                                            ⁢                      R                                                        ⁢                                      xe2x80x83                                                                                        xe2x80x83                                    ⁢                                                            g                                                                        xe2x80x83                                                ⁢                        m1                                                              +                                          j                      ⁢                                              xe2x80x83                                            ⁢                                              ω                        [                                                  xe2x80x83                                                ⁢                                                                              C                                                                                          xe2x80x83                                                            ⁢                              gs1                                                                                -                                                                                    (                                                              xe2x80x83                                                            ⁢                                                                                                                                    xe2x80x83                                                                    ⁢                                                                      g                                                                                                                  xe2x80x83                                                                            ⁢                                      m1                                                                                                                                                                                                            xe2x80x83                                                                    ⁢                                                                      g                                                                                                                  xe2x80x83                                                                            ⁢                                      m2                                                                                                                                                                  )                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          C                                                                                                xe2x80x83                                                                ⁢                                gs2                                                                                                              ⁢                                                      xe2x80x83                                                    +                                                                                    (                                                              xe2x80x83                                                            ⁢                                                                                                ω                                  ⁢                                                                      xe2x80x83                                                                    ⁢                                                                      C                                                                                                                  xe2x80x83                                                                            ⁢                                      gs2                                                                                                                                                                                                            xe2x80x83                                                                    ⁢                                                                      g                                                                                                                  xe2x80x83                                                                            ⁢                                      m2                                                                                                                                                                  )                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          C                                                                                                xe2x80x83                                                                ⁢                                gs1                                                                                                                                    ]                                                                                                                                                    (        1        )            
In the case that the transistors M1 and M2 have the same characteristics, equations Cgs1=Cgs2=C and gm1=gm2=g are satisfied. Thus, the equation (1) is expressed as follows:                               Z          ⁢                      xe2x80x83                    =                      xe2x80x83                    ⁢                                    1                                                xe2x80x83                                ⁢                                  1                  +                                                            (                                              xe2x80x83                                            ⁢                                              ω                                                                              xe2x80x83                                                    ⁢                                                      ω                                                                                          xe2x80x83                                                            ⁢                              T                                                                                                                          )                                        2                                                                        ·                          xe2x80x83                        ⁢                                                            xe2x80x83                                ⁢                                                      1                                                                  xe2x80x83                                            ⁢                      g                                                        +                                      j                    ⁢                                          xe2x80x83                                        ⁢                    ω                    ⁢                                          xe2x80x83                                        ⁢                                          CR                                                                        xe2x80x83                                                ⁢                        g                                                                                                                        1                +                                                      j                    (                                          xe2x80x83                                        ⁢                                          ω                                                                        xe2x80x83                                                ⁢                                                  ω                                                                                    xe2x80x83                                                        ⁢                            T                                                                                                                )                                    3                                                                    ⁢                  xe2x80x83                                    (        2        )            
where xcfx89T=g/C.
The common denominator of the right term on the right side of the equation (2) is 1+j(xcfx89/xcfx89T)3. Under the condition where:                                           (                          ω                              ω                T                                      )                    3                ⪡        1                            (        3        )            
approximation can be obtained as follows:                               Z          ≈                                                    xe2x80x83                            ⁢                                                1                                                            xe2x80x83                                        ⁢                    g                                                  +                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                                      CR                                                                  xe2x80x83                                            ⁢                      g                                                                                                          1              +                                                (                                      xe2x80x83                                    ⁢                                      ω                                                                  xe2x80x83                                            ⁢                                              ω                                                                              xe2x80x83                                                    ⁢                          T                                                                                                      )                                2                                                    ⁢                  xe2x80x83                                    (        4        )            
Moreover, under the condition where:                                           (                          ω                              ω                T                                      )                    2                ⪡        1                            (        5        )            
the condition of the expression (3) is also satisfied, and the expression (4) can be approximated as follows:                     Z        ≈                              1                                          xe2x80x83                            ⁢              g                                +                      j            ⁢                          xe2x80x83                        ⁢                          ω              ⁡                              (                                  CR                                                            xe2x80x83                                        ⁢                    g                                                  )                                                                        (        6        )            
Therefore, under the condition of the expression (5), the active inductor shown in FIG. 23 can be approximated by a circuit having a resistance component (1/g) connected in series to an ideal inductor CR/g as shown in FIG. 26.
Even when employing the equivalent circuit shown in FIG. 24 in which a resistive component in the transistors M1 and M2 is ignored, a series resistive component exists as shown in the expression (6). This shows that loss is in principle unavoidable in the active inductor 200.
A first aspect of the present invention is directed to an active inductor comprising first and second field effect transistors, each having a source, a gate and a drain, wherein the drain of the first field effect transistor is connected to the source of the second field effect transistor, the gate of the first field effect transistor is connected to the drain of the second field effect transistor with no active element interposed therebetween. The active inductor further comprises a feedback path provided between the source of the first field effect transistor and the gate of the second field effect transistor, wherein the gate and the source of the second field effect transistor serve as two ports of the active inductor.
A second aspect of the present invention is directed to an active inductor comprising first to third field effect transistors, each having a source, a gate and a drain, wherein the drain of the first field effect transistor is connected to the source of the second field effect transistor and the gate of the third field effect transistor, the gate of the first field effect transistor is connected to the drain of the second field effect transistor, the gate of the second field effect transistor is connected to the drain of the third field effect transistor. The active inductor further comprises a feedback path provided between the source of the first field effect transistor and the source of the third field effect transistor, the gate and the source of the third field effect transistor serve as two ports of the active inductor.
A third aspect of the present invention is directed to an active inductor comprising first to fourth field effect transistors, each having a source, a gate and a drain, wherein the drain of the first field effect transistor is connected to the source of the second field effect transistor, the gate of the third field effect transistor and the gate of the fourth field effect transistor, the gate of the first field effect transistor is connected to the drain of the second field effect transistor, the gate of the second field effect transistor is connected to the drain of the third field effect transistor, the source of the third field effect transistor is connected to the source of the fourth field effect transistor. The active inductor further comprises a feedback path provided between the source of the first field effect transistor and the drain of the fourth field effect transistor, wherein the source and the drain of the fourth field effect transistor are set at the same potential, current into a junction between the drain of the fourth field effect transistor and the feedback path and current into a junction between the source of the fourth field effect transistor and the source of the third field effect transistor are set to flow independently of each other, the gate and the drain of the fourth field effect transistor serve as two ports of the active inductor.
Preferably, according to a fourth aspect of the present invention, the active inductor of the second or third aspect further comprises a resistor connected in series to the two ports of the inductor.
Preferably, according to a fifth aspect of the present invention, the active inductor of any one of the first to fourth aspects further comprises a field effect transistor for feedback provided on the feedback path, having a source connected to the source of the first field effect transistor, a gate and a drain connected to each other and to one of the two ports of the inductor.
As has been described, the active inductor according to the first to third aspects reduce the series resistance component of the active inductor. Further, as in the active inductor according to the fifth aspect, a field effect transistor for feedback having its gate and drain connected to each other may be provided on the feedback path.
Since a negative resistance is generated particularly in the active inductor according to the second and third aspects, a resistance can be connected in series from the outside, thereby controlling the series resistance as in the active inductor according to the fourth aspect.
Particularly in the active inductor according to the third aspect, at least one of the third and fourth field effect transistors is controlled, so that the characteristics, e.g., the band, is controlled.
An object of the present invention is to provide an active inductor with minimized loss. Another object of the present invention is to provide an active inductor with a wide range, i.e., a broad band, which is a predetermined range which an inductance falls within.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.